Sulfur compounds such as hydrogen sulfide and methyl mercaptan are contained in trace amounts in volatile components of food materials, including vegetables such as cabbages, tomatoes and potatoes, fruits such as passionfruits, grapes and grapefruits, and green tea or roasted coffee beans. These volatile components, including sulfur compounds, are collectively referred to as "an aroma" in this specification.
Gases containing a volatile aroma including hydrogen sulfide and methyl mercaptan (hereinafter called "aroma-containing gases or an aroma-containing gas" in the context) evolve during steps for treating or processing food materials such as coffee, tea or citrus fruits. For the production of instant coffee or liquid coffee, for example, an aroma-containing gas occurs during a treating or processing step, e.g., when coffee beans are roasted; when roasted coffee beans are ground by means of a mill; when water (normally 80 to 100.degree. C.) is poured over ground coffee beans to take water-soluble substances, with the result that gases mainly containing air or carbon dioxide, which lie in the gaps between the ground coffee beans or exist in the cells of the beans, are purged with water; or when steam is blown into ground coffee for steam distillation.
It is customary practice to recover these aroma-containing gases, and add aroma components to an aqueous solution containing a coffee extract before drying, or coffee powder after drying, thereby enhancing the flavor of coffee drinks. This practice is described, for example, in U.S. Pat. Nos. 3,077,405 and 3,769,032.
A method is also known in which the aroma-containing gas taken from coffee is condensed using a heat exchanger with liquid nitrogen as a refrigerant so that it is recovered as an aroma-containing frost, and the recovered frost is used for addition to triglycerides or edible oils or fats (U.S. Pat. Nos. 3,021,218 and 4,551,344). For the production of soluble powders or drinks from food materials other than coffee, it has been attempted to recover aroma components during the processing step by various methods.
If the recovered aroma contains large amounts of sulfur compounds such as hydrogen sulfide and methyl mercaptan, the step of concentrating this aroma may give rise to an irritating foul smell which is different from a fragrance inherent in the food material. As an attempt at improving this problem in quality, the following techniques are disclosed: U.S. Pat. No. 2,875,063 describes a method of removing hydrogen sulfide by contacting an aroma-containing gas, recovered from a food material, with spiral cuttings of copper. Japanese Laid-Open Patent Publication No. 123109/93 describes a method which comprises mixing a small amount of an edible oil or fat with a frost that has been obtained by the condensation of the aroma-containing gas with liquid nitrogen, thereby transferring the smelly components to the edible oil or fat to remove them. With the former method, however, methyl mercaptan, the substantial cause of the smell, is not removed, and it is required to treat an organic waste liquor produced during the regeneration of the copper material. The latter method is difficult to carry out commercially, because of increased costs involved for the losses of the edible oil or fat and the aroma components, as well as the complexity of the procedure. U.S. Pat. No. 5,008,125 describes a method of treating a liquid condensate of volatile components, recovered from coffee, with microporous carbon, but its invention clearly differs from the present invention in the object to be treated and in the requirements for the adsorbent used.
An adsorption operation using activated carbon or the like is generally used for the removal of various smelling components. For removal of sulfur compounds, in particular, there are disclosed methods of promoting the adsorption of sulfur compounds by impregnating the surface of ordinary activated carbon with a metal salt of Mn, Cu, Cr, Ni or Fe (Japanese Patent Publication Nos. 137089/78, 4727/85 and 729/87). It has been disclosed that sulfur compounds are effectively removed by the use of activated carbon impregnated with iodine or bromine or a compound thereof (Japanese Patent Publication Nos. 20297/80 and 2368/82). These methods of removal using impregnated activated carbon, however, involve removal based on a chemical reaction between the impregnant and the sulfur compounds, and may present with a reaction with aroma components other than the sulfur compounds, deteriorating the odor. Furthermore, the activated carbon, the carrier, has a broad pore distribution, thus adsorbing and removing aroma components other than the sulfur compounds, resulting in the elimination of the odor itself.